A new chitinozoan assemblage from the Middle Devonian Los Monos Formation (sub-Andean basin, southern Bolivia) and its biozonal implications for Western Gondwana

Chitinozoans recovered from one section of the Middle Devonian Los Monos Formation in the TCB X-1001-Tacobo borehole, sub-Andean basin of Bolivia, have been analysed. Eleven from the eighteen processed cutting samples yielded specimens that allowed taxonomic study. Eleven genera and thirty-five chitinozoan species were identified from the Los Monos Formation with four of them recorded for the first time in Western Gondwana. Ancyrochitina biconstricta, Ancyrochitina parisi, Angochitina galarzae and Ramochitina boliviensis are among the most relevant taxa restricted to Western Gondwana that support the affinity with this paleocontinent. One new species, Lagenochitina tacobensis sp. nov. is described, and Ramochitina candelariaensis sp. nov. (n. n.) is formally erected. The chitinozoan assemblage reinforces the late Eifelian–middle Givetian age previously proposed by organic-walled phytoplankton and miospores for this section of the TCB X-1001-Tacobo borehole. A new local chitinozoan biozonation based on the chitinozoan assemblages is proposed and a revision of the current chitinozoan biozonation for Western Gondwana and Bolivia is recommended.


Introduction
The sub-Andean basin of southern Bolivia constituted part of Western Gondwana throughout the Devonian.The geological evolution of this Gondwana margin during the Phanerozoic is defined by a tectonically active and structurally complex framework that allowed the sedimentary infill to be divided into several overlapping sedimentary basins [1].One of these basins developed during the Silurian through the Devonian with a large volume of siliciclastic sediments deposited by relative sea-level changes.The vastness and complex tectonic evolution of these Phanerozoic basins have made biostratigraphical and palynological studies extremely important to understand the stratigraphical configuration of Devonian sediments.Most of the previous biostratigraphical studies in these basins are based on palynological data due to the scarcity of other useful fossil groups [2].The complete list of previous palynological studies in the area can be found in Garcı ´a Muro et al. [3].
The TCB X-1001-Tacobo borehole was drilled in the eastern sector of the sub-Andean ranges known as 'the Foothills'.The samples studied come from the Icla, Huamampampa and the Los Monos formations.The phytoplankton and miospores were described by Garcı ´a Muro et al. [3], thus the present study focuses on the chitinozoans from the Los Monos Formation, which is the other important palynological marine group.
Chitinozoan studies are very important in Palaeozoic marine strata, as they provide excellent data for detailed biostratigraphical studies, together with phytoplankton and miospores.The main advantages of this fossil group are their wide palaeogeographical distribution, rapid morphological evolution and the variety of marine sedimentary facies from which they can be recovered (e.g.[4][5][6]).
Since previous records of detailed chitinozoan taxonomy from the Los Monos Formation in Bolivia are extremely scarce [7][8][9], this work aims to provide a new fully-detailed record of their taxonomy in order to contribute to a better knowledge of the Devonian chitinozoan assemblages of Western Gondwana.
Another important goal of this research is to accurately constrain the age of the Los Monos Formation in the Tacobo borehole, previously based on miospore and organic-walled phytoplankton assemblages.A chitinozoan Local Biozonation is proposed for the Los Monos chitinozoan assemblage to be compared with other coeval assemblages of South America and correlated to both the regional chitinozoan biozonations for Bolivia [9] and Western Gondwana [10].

Geological setting and stratigraphy
The TCB X-1001-Tacobo borehole was drilled in the eastern sector of the sub-Andean ranges (the 'Foothills'), in southern Bolivia (Fig 1).Previous palynological work on this well was published in detail by Garcı ´a Muro et al. [3] in which the marine deposits of the Icla, Huamampampa and Los Monos formations were analysed and the corresponding geology and stratigraphy were extensively described.
During the Devonian, the sub-Andean basin of Bolivia formed part of Western Gondwana and was situated about 60˚S [2].This high palaeolatitude and shallow marine deposits suggested cold sea conditions and a palaeobathymetry of no more than 200 m [11,12].Although the middle and upper Palaeozoic tectonic setting along the Bolivian basins is still under debate (see [3]), the sedimentary infill is considered to be made up of several overlapping sedimentary basins.
The depositional system during the Silurian-Devonian was influenced by relative sea-level changes resulting in the deposition of a large volume of clastic sediments separated by flooding surfaces.These surfaces allowed Starck et al. [13,14] to propose three supersequences: Cinco Picachos, Las Pavas, and Aguaragu ¨e.The Los Monos Formation (Fig 2 ) is the basal unit of the Aguaragu ¨e supersequence [1, [13][14][15][16] and is considered as the main source of oil and gas in the area.
The Los Monos Formation comprise dark grey to black laminated shales, with fine-grained and muddy sandstones [17].These rocks were deposited mostly in a shallow to outer shelf marine palaeoenvironment and some local intervals seems to be nearshore or transitional with the overlying unit [18,19].
Previous studies dated the Los Monos Formation as late Eifelian [17,20], late Eifelian-late early Givetian to late Givetian [21] and early-middle Givetian to Frasnian [7].Garcı ´a Muro some chitinozoan specimens distorting or breaking the walls of the chamber and/or the neck.The preservation was predominantly good, although all specimens were flattened and some of them were broken or lost processes and ornamentation (e.g.removal of the basal processes and/or long spines), hindering their identification.
The slides are housed in the palaeopalynological collection of IANIGLA, CCT CONICET Mendoza, Argentina.

Chitinozoan assemblage
A total of eleven genera and thirty-five chitinozoan species were identified from the Tacobo borehole, with nineteen being retained in open nomenclature.Five of the taxa are recorded here for the first time in Western Gondwana, and nine are restricted to Western Gondwanatwo new species are formally described herein.
The highest abundances (9.2 to 17.7 specimens per gram of rock) were recorded in the middle part of the studied interval of the Los Monos Formation, between 4830 to 5170 m depth (samples 9166 to 9160), particularly from depths 4870 and 4975 m (samples 9161 and 9162) with 19 and 21 species respectively.Meanwhile, in the upper and lower part, the diversity decreases to 5 and 2 chitinozoan species respectively.This relatively high chitinozoan abundance drops significantly in the uppermost and lowermost part of the studied section in samples 9116 and 9158 (0.6 to 3.4 specimens per gram of rock) (Fig 3).

Systematic palaeontology
The classification used herein follows the scheme proposed by Paris et al. [23], and the system of open nomenclature follows the recommendations of Bengtson [24].
The biometric measurements were taken in their raw form with no correction factor applied for flattening.Although our specimens are entirely compressed, raw data is adopted herein as correction factors are subjective and can be applied to the measurements herein by subsequent workers if desired.
The principal measurements, recorded in micrometres (μm), are: L = total length; ln = length of the neck; lc = length of the chamber; D = maximum diameter of the chamber; dn = diameter of the neck, and da = diameter of the aperture.
Diagnosis: Belonechitininae with cylindro-conical vesicle chamber and randomly distributed simple and very short spines and tubercles.Description: The vesicle chamber is cylindro-conical, with a flat to slightly convex base and blunt basal margin.The flanks are straight with an inconspicuous to slight flexure and a very weak to absent shoulder.The neck is cylindrical and occasionally flares slightly towards the aperture, which occupies approximately one-third (30%) of the total length of the vesicle.Randomly distributed granules and simple spines (Fig 4A') cover the whole vesicle with even density.The length of the ornamentation varies between 0.7 and 10 μm, and they are spaced between 0.3 and 1.7 μm apart on the vesicle surface.The basal features were only observed in one specimen, as the bases are flattened in all others, and no mucron or other distinguishing characteristics were observed.
Remarks: The specimens assigned to this taxon herein are retained in open nomenclature, but display the main characteristics of the genus Belonechitina; Conochitinidae with randomly distributed simple spines.Ottone [30] described Belonechitina holfeltzii, an Argentinean species, as a possible transition from the genus Conochitina to Angochitina which is similar in shape to our taxon.However, the ornamentation of B. holfetzii is comprised of spines branching distally, whereas our specimens have simple spines and tubercles.
Description: The vesicle chamber is sphero-conical, with a convex base and broadly rounded basal margin.The flanks are weakly concave, with a marked flexure.The long neck is cylindrical and occupies one half (50%) of the total length.The aperture flares, and is surrounded by a slightly denticulated collarette.The vesicle surface is smooth, with some isolated small tubercules (less than 2 μm in height).No basal features are discernible, as the base is flattened inwards in the specimen.
Remarks: The single specimen assigned to Sphaerochitina ricardi Dı ´ez & Cramer, 1978 [32] herein bears a similar vesicle shape and neck proportion to the original description of the species.Dı ´ez & Cramer [32] mentioned surface ornamentation composed of small granules or spines widely spaced which is similar to the isolated small tubercles of the Tacobo specimen.However, our specimen is slightly larger than previous records of this species, except those of Winchester-Seeto & Carey [34], assigned to Angochitina cf.Sphaerochitina ricardi which have a total length similar to that of the Los Monos Formation.We agree that those specimens of Angochitina cf.Sphaerochitina ricardi indeed are not assignable to S. ricardi.
Description: The vesicle chamber is ovoid, with a very weak to inconspicuous basal margin and a convex-rounded base.The flanks are convex, and display a distinct flexure with a gentle shoulder.The neck occupies less than a half of the total length (45%), is cylindrical, and on some occasions may be slightly flared towards the aperture.The vesicle wall is smooth, without any ornamentation.The bases of all specimens are flattened inwards, and as such basal features cannot be discerned.
Remarks: The Tacobo borehole specimens strongly resemble Lagenochitina amottensis Grignani & Mantovani, 1964 [35], in vesicle shape and measurements.Although the original description of the species is based on light microscope images, the similar outline and description allow for the confident assignation of the species.The only difference is the length of the neck which was recorded in the type material as being half of the total length, but in our specimens reach a maximum of 45% (4/9) occupied by the neck.
Occurrence: Global distribution: Lagenochitina amottensis was recorded in Northern Gondwana from the Middle and Upper Devonian of the OUM DOUL 1 borehole, Morocco [35], and Givetian of the Jinbo Formation, South China [36].There are no previous records from Euramerica or Western Gondwana.
Description: The vesicle chamber is ovoid, with a broadly rounded to inconspicuous basal margin and a convex base.The flanks are convex, with a gentle flexure and weakly developed shoulders.The cylindrical neck occupies approximately one-third (30%) of the total length, terminating in a flaring collarette.The surface of the vesicle is predominantly smooth, although some granules may occur.Basal features are not discernible, due to inward flattening of the base of the specimen.

Remarks:
The specimen assigned to Lagenochitina sommeri (Lange, 1952) [37], herein matches in dimensions and general outline with the type material.Lange placed this species within the genus Desmochitina, though it was later assigned by Ottone [30] and Grahn and Melo [38,39] to Lagenochitina, due to the clear differentiation of the neck and body and glabrous surface.
Diagnosis: Lagenochitina species with an ovoid elongated vesicle shape with a short neck occupying one-quarter of the total length and smooth surface.
Description: The vesicle chamber is ovoid elongated, with a convex base and well-rounded basal margin.The flanks are generally convex but may seem to be straight in some specimens.The flexure is weak and the shoulder is inconspicuous to absent.The cylindrical neck covers approximately one-quarter (30%) of the total length, and the aperture bears a non-flaring slightly denticulate collarette.The vesicle wall is smooth with some isolated tubercules.No mucron or basal features are observed.
Remarks: Lagenochitina tacobensis sp.nov.differs from Lagenochitina claviformis Rauscher & Doubinger, 1967 [40] in that the latter has a clavate vesicle chamber and longer total length.Lagenochitina cylindrica Eisenack, 1931 [25] is larger than L. tacobensis sp.nov.and the vesicle shape is cylindrical.Lagenochitina longiformis Obut, 1995 [41], has a similar outline, but the neck is much longer than our species and the overall size is larger.Lagenochitina pirum Achab, 1982 [42] has a conical vesicle shape.Occurrence: Lagenochitina tacobensis sp.nov. is recorded from the Tacobo X-1001 borehole in southern Bolivia, assigned to the early Givetian.The holotype is recorded in sample 9161 (4870 m).
Description: The vesicle chamber is cylindroconical to subconical, with a slightly convex base and well-rounded basal margin.The flanks are straight to slightly convex, and display a subtle flexure with a weakly developed shoulder.The cylindrical neck covers approximately one-half (50%) of the total length, and flares slightly towards the aperture.The vesicle wall is smooth with some isolated tubercules.No mucron or other basal features are observed.
Remarks: Taugourdeau [43] described and illustrated this species with only light microscope images.He describes the specimens as completely transparent, indicating that it is a diagnostic characteristic.However, it is a preservational feature which cannot be considered as diagnostic.The vesicle outline and measurements are similar to the type material.The length of the neck of the Tacobo specimens is half of the total length, thus slightly shorter than that of the type material.
Occurrence: Global distribution: Lagenochitina vitrea was recorded in Northern Gondwana from the middle and upper Llandovery strata of wells in the Sahara [43].In Iberia (Armorica terrane) was recorded from the upper part of the Formigoso Formation and the lower part of the San Pedro Formation, both of Silurian age [44,45].There are no previous records from Western Gonwana.

Description:
The vesicle chamber is subconical to subcylindrical, with a convex base and distinctly rounded basal margin.The flanks are straight to weakly concave, and display an inconspicuous to subtle flexure.The shoulder is generally absent, but on some specimens a subtle inflexion is apparent.The short neck is cylindrical, and forms approximately one-quarter (25%) of the total length.The aperture is bordered by a slightly flaring collarette, which is simple to gently denticulated.The vesicle surface is smooth to slightly granulate.A mucron is observed at the centre of the base on some specimens, as the only feature present at the base.
Remarks: Although the Tacobo specimens are similar to Lagenochitina pirum (Achab, 1982) [42] in the general vesicle outline, they are smaller in size and with a smooth vesicle surface instead of rugose.This species was previously recorded only from the Early and Middle Ordovician.Due to the large time interval between the two records (Ordovician and Devonian), the Tacobo specimens could represent a new species, so they are doubtfully assigned to Lagenochitina pirum.
Occurrence: Global distribution: Lagenochitina pirum (Achab, 1982) [42] has been described from the Early Ordovician in Euramerica by Achab and Achab and Asselin [42,46,52,53], and from the middle Ordovician in Eastern Gondwana by Chen et al. and Tang et al. [48,50,51].There are no previous records of this species in the Silurian or the Devonian.
Diagnosis: Lagenochitina species with a wide neck almost equal to the diameter of the chamber (Dp:dn = 1).
Description: The vesicle chamber is elongated ovoid to subcylindrical, with a convex base and rounded basal margin.The flanks are straight to slightly convex, with a subtle flexure.The neck is short and wide, occupying approximately one-quarter (25%) of the total length.Its shape is cylindrical and non-flaring at the aperture.The vesicle surface is smooth to weakly granulated.No basal features are discernible, as a consequence of inward flattening of the base.Remarks: Although the genus Euconochitina is also recognized by the gentle flexure and lack of a shoulder, the present specimens are assigned to Lagenochitina due to the elongated ovoid vesicle shape, differentiated neck and body chamber, and smooth surface.It differs from other species of Lagenochitina by the width of the neck which is almost equal to the diameter of the chamber.The specimens recovered are scarce, completely flattened and mostly fractured, and as such, they are retained in open nomenclature.
Occurrence: Lagenochitina sp.A is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Material: One moderately well-preserved specimen was observed and measured from sample 9162 (4975 m) (Table 9).
Diagnosis: Lagenochitina species with a conical chamber, flat base and short neck.Description: The vesicle chamber is conical, with a flat base and broadly rounded basal margin.The flanks are straight and display no distinct flexure.The neck is short, occupying approximately one-fifth (20%) of the total length, with a gently flaring denticulated collarette.The vesicle surface is generally smooth, with some isolated granules.No basal features are discernible, as a consequence of inward flattening of the base.
Remarks: The specimen assigned to this genus displays the main characteristics of Lagenochitina, but due to the paucity of material and lack of distinguishing features for identification at species level, it is retained in open nomenclature.
Occurrence: Lagenochitina sp.B is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.

Description:
The vesicle chamber is conical, with a concave base inferred from its inward flattening.The flanks are straight to weakly concave with a gentle flexure.The long neck is cylindrical and occupies approximately one-half (50%) of the total length.The aperture is outlined by a flaring, slightly denticulated collarette.The surface of the vesicle is smooth with some occasional small tubercules (less than 2 μm in height).The base bears a single complete carina which is c. 13 μm wide.The carina seems to have been positioned normal to the longitudinal axis, however, due to flattening it appears at a lower angle.
Occurrence: Global distribution: Cyathochitina campanulis has only been recorded by Boneham & Masters [54] from Euramerica, in the Silurian Osgood Member of the Salamonie Dolomite (Indiana, USA).

Description:
The vesicle chamber is conical to sub-ovoid, with a slightly convex base and clearly rounded basal margin.The flanks are straight to weakly convex, with a gentle flexure that in some occasions is marked.The shoulder is weakly developed to absent.The cylindrical neck occupies approximately two-fifths (40%) of the total length and flares towards the simple aperture.The surface of the vesicle is covered with randomly distributed simple small spines up to 3 μm over the neck, that increase in size towards the base of the vesicle reaching up to 18 μm in length (Fig 6I').In some specimens, the surface is highly eroded and broken spines may look like small tubercles-when a detailed observation is made, however, the scars and broken spines can be recognized clearly.No mucron is present.
Remarks: The specimens assigned to Fungochitina pilosa herein strongly resemble those originally described by Collinson & Scott [57] in both vesicle shape and measurements.Some specimens display areas denuded of spines and which appear to be smooth, although attachment scars can be recognized.The spiny ornamentation normally decreases in size from the base to the neck until reaching the size and shape of granules.
This species was originally considered as belonging to the genus Sphaerochitina, but was reassigned to Fungochitina by Paris [27] based on its conical, almost "fungic", vesicle shape and randomly distributed spines.Sphaerochitina schwalbi Collinson & Scott, 1958 [57], is very similar to F. pilosa, but is differentiated by a slightly different vesicle shape and size.These are features that allow for a clear separation of both species, and therefore we consider S. schwalbi a junior synonym of F. pilosa.To further compound matters, F. pilosa is considered to be a wastebasket for some authors, but the strong similarities of our specimens to the original description of the species allows for the confident designation of the Tacobo specimens herein to F. pilosa.
Occurrence: Global distribution: Fungochitina pilosa was recorded in Euramerica from the Middle Devonian Cedar Valley Formation and Wapsipinicon Formation, USA [57,59,60] and in the Middle Frasnian from Boulonnais, USA [58].Northern Gondwana: recorded from the Givetian of the Me ´derba and Alrar formations, southeastern Algeria [62].Western Gondwana: from the Middle-Late Devonian of the São Domingos and Lima formations, Parana ´Basin of Brazill and Paraguay [64]; from the Middle Devonian of the Erere ´Formation, Amazonas Basin, Brazil [65]; from Devonian of the San Antonio X-1 borehole, Tarija Basin, Argentina [66].
Fungochitina pilosa Collinson & Scott, 1958) is recorded herein from the early and middle Givetian in the TCB X-1001-Tacobo borehole.
Description: The vesicle chamber is ovoid, with an inconspicuous basal margin and convex base, although in some specimens due to inward flattening the base seems to be concave.The flanks are clearly convex, with marked flexure and a very weak to inconspicuous shoulder.The neck is cylindrical and displays mostly non-flaring slightly denticulate collarette, although in some instances the collarette can be weakly flaring.The ornamentation over the vesicle wall comprises simple long hairs or spines (from 2 to 10 μm long, and 0.5 to 2 μm in diameter) with even distribution over the entire surface (Fig 5O ').The spacing between spines is c. 1 to 5 μm.The base bears the same ornamentation as the rest of the vesicle.
Remarks: Specimens from the Tacobo borehole that are assigned confidently to Angochitina galarzae resemble the original description of Ottone [30] in terms of vesicle shape, dimensions and ornamentation.The specimens assigned uncertainly display a slightly longer neck, reaching almost one-half of the total length and a slightly flaring collarette which is not observable in the type material.Nevertheless, Ottone [30] illustrated some variation in the length of the neck of his specimens, suggesting that this feature may vary from specimen to specimen.Another difference is the gentle flaring collarette at the end of the aperture discernable in some specimens.This is a subtle variation and it may be caused by the flattening of the material, as some fractures can be discernible at the aperture of these specimens.These two slightly different characteristics are not distinct enough to allow for splitting of the taxon, hence all specimens are assigned herein to Angochitina galarzae.
Description: The vesicle chamber is ovoid, with a well-rounded basal margin and convex base.The flanks are convex, and a gentle flexure with an inconspicuous shoulder occurs.The neck is short, covering approximately one-fifth (20%) of the total length and is fringed by a simple thin-walled collarette at the aperture.The vesicle surface is covered by simple hairs or spines (length of the spines 3 to 5 μm and diameter 0.5 to 1 μm), which are densely and randomly distributed (space between spines c. 2 to 4 μm) (Fig 6C ').No basal features could be recognized as a consequence of inward flattening.Remarks: Angochitina capillata Eisenack, 1937 [68] is a well-recorded species from the Ordovician and Silurian, but they differ from the Tacobo Middle Devonian specimens by several characteristics.The total length is similar in the entire specimen, although the measurement ratios are quite different.The neck in the Devonian specimens is significantly shorter, occupying one-fifth (20%) of the total length (Lp: Ln = 4:1), while in the older species this ratio normally is Lp: Ln = 2:1.The vesicle chamber from the Devonian specimens is mainly ovoidspherical, as the older species is mainly ovoid elongated to subconical on some occasions.These two features are consistent and evident in both previous records of Angochitina capillata, and also in the Tacobo material.As the overall shape and morphological ratios are very different from older Angochitina capillata specimens, Middle Devonian species may well prove to be a separate taxon.Only one specimen from our material displays these noticeable characteristics, however, more material would be necessary to confirm a differentiation into two separate species-as such, it is retained in open nomenclature herein.
Occurrence: Global distribution: Angochitina capillata Eisenack, 1937 [68] has been extensively reported from Middle Ordovician to Devonian strata on all palaeocontinents.Although the Middle Devonian records are scarce, in Euramerica Urban & Newport [60] recorded the species from the Wapsipinicon Formation (Iowa, USA), and in Iberia (Armorica terrane) Askew & Russell [33] recorded it from the early Givetian of the Naranco, Huergas and Gustalapiedra formations.
Description: The vesicle chamber is elongated ovoid, with an inconspicuous basal margin and convex base.The flanks are straight to slightly convex, with a gentle to inconspicuous flexure.The neck is cylindrical with a thin-walled collarette which is simple and subtly flaring, and occupies approximately two-fifths (40%) of the total length.The vesicle surface is covered with randomly distributed, simple hairs or spines (1 to 7 μm long, 0.5 to 2 μm diameter at the base) spaced between 3 and 8 μm apart.No basal features could be recognized as a consequence of inward flattening of the base.
Remarks: Angochitina elongata was described by Eisenack [25] as an Angochitina species with a slender and elongated chamber that is generally longer than the neck with very short spines.A neotype was described by Eisenack [72], with denser ornamentation, a slightly different vesicle shape, and being less elongate than in the original description.Laufeld [69]  distinguished two slightly different populations in Gotland, which he interpreted as intraspecific variations.Angochitina cf.elongata is very similar to Angochitina elongata Eisenack, 1931 [25] but the former differs in having shorter spines and tubercules that are sparsely arranged, and smaller in size.The Tacobo specimens herein also have a weaker flexure and no shoulder.These two characteristics make the transition between the chamber and the neck less evident and create a slightly different general outline to the type material.The specimens assigned to Angochitina cf.elongata herein are similar to specimens of Angochitina elongata described by Nestor [73] from the Ludlow part of the Kaugatuma GI borehole (Estonia).
Occurrence: Global distribution: There are no previous records of Angochitina elongata Eisenack, 1931 [25] in the Middle Devonian since it has been exclusively recorded worldwide from the Late Silurian.
Diagnosis: Angochitina species with an elongated ovoid vesicle chamber, a short cylindrical neck and small spines randomly distributed all over the vesicle surface.
Description: The vesicle chamber is an elongated ovoid, with a convex base and inconspicuous basal margin.The flanks are straight to slightly concave, with a weak flexure.The neck is cylindrical and occupies approximately one-quarter (25%) of the total length, and the aperture is fringed by a non-flaring collarette.The ornamentation consists of simple spines that cover the entire vesicle surface with a length between 1 and 4 μm, a diameter from 0.5 to 1 μm, and which are spaced approximately 1 to 4 μm apart.The specimen displays inward flattening at the base, and therefore no basal features can be discerned.
Remarks: The specimen assigned herein to Angochitina sp.resembles Angochitina cf.elongata in vesicle shape and size, but it has a much shorter neck and denser distribution of the spines.Other known Angochitina species have an elongated vesicle shape and long neck or have a short neck with an ovoid body.In the studied material there is only one specimen with these characteristics, therefore it is retained in open nomenclature.
Occurrence: Angochitina sp. is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Diagnosis: Angochitininae with a cylindroconical vesicle chamber, and the vesicle wall partially covered by a randomly distributed net.
Description: The vesicle chamber is cylindroconical to subovoid, with a broadly rounded basal margin.The flanks are straight to subtly convex, with a gentle flexure and inconspicuous to absent shoulder.The neck is cylindrical with an aperture that is slightly flaring and occupies approximately one-third (30%) of the total length.The ornamentation comprises a stout net that partially covers different parts of the specimen (Fig 6H ').In some specimens, it occurs at the basal margin or/and surrounding the neck, but it also may be randomly distributed over the vesicle.The spaces between the net are covered by simple and on some occasions multirooted and bifurcated spines.No basal features could be discerned as a consequence of inward flattening.

Remarks:
The specimens assigned to Muscochitina?sp.A herein bears a unique ornamentation.The vesicle wall is partially covered by a stout net that on some occasions is aligned parallel to the longitudinal axis but also may be randomly distributed.The position of the ornamentation is also highly variable; it can be located only in one place or in multiple areas on the vesicle.It can occur over the neck, over the vesicle chamber or over the basal margin and any combination thereof.
A confident assignation of these specimens to the family Lagenochitinidae is due to a clear differentiation through the flexure of the neck and body.However, the genus assignation is doubtful due to the stout net distributed partially over different parts of the surface.The scarce and poorly preserved specimens of the Tacobo Borehole prevent the creation of a new genus.
Diagnosis: Angochitininae with an ovoid vesicle chamber with the vesicle wall completely covered by a net.
Description: The vesicle chamber is ovoid, with a rounded basal margin and a flat to slightly convex base (inferred due to flattening).The flanks are convex and a gentle flexure with a slight shoulder occurs.The vesicle surface is covered by a stout net which is better developed near the base.The basal margin bears two different types of processes: the first is short (13 μm approximately), triangular, wide at the base (between 8 and 14 μm) and terminates sharply at the distal end.The second type, although broken, appears to be a thinner and tubular process (Fig 6K').
Remarks: Only one specimen was assigned to Muscochitina?sp.B herein.Although the vesicle wall ornamentation is very similar to that of Muscochitina?sp.A, the net covers the entire surface, and is better developed near the base than over the neck.It also differs from Muscochitina?sp.A in the differentiated basal processes.The single specimen is provisionally assigned to Muscochitina based on the surface ornamentation.18).
Description: The vesicle chamber is conical to subconical, with a broadly rounded basal margin and a flat to slightly convex base.The flanks are straight and, on some occasions, slightly convex with a weak to absent flexure, and inconspicuous shoulder.The neck is cylindrical and occupies one-third (35%) of the total length.The aperture is fringed with a simple and non-flaring collarette.The vesicle surface is covered with simple and bifurcated spines which are aligned parallel to the longitudinal axis.The space between spines in a row is 5 to 10 μm, and their dimensions vary between 2 to 30 μm in length and 0.5 to 5 μm in width.No mucron or other basal features are present.
Remarks: Specimens assigned to Ramochitina autasmirimense Grahn & Melo, 2004 [65] herein bear the same diagnostic characteristics of the species.They only differ in having both simple and some bifurcated spines, which were not mentioned or described by the authors.However, Grahn and Melo [65 pl. 1, fig.8] illustrated one specimen that shows some bifurcated spines.With this characteristic being clearly developed in our material, we suggest that bifurcated spines, as well as simple spines, should be added to the diagnostic features of the species.

Description:
The vesicle chamber is conical to subconical, with a rounded well-defined basal margin and a flat base.The flanks are generally straight, though on some occasions are slightly convex with a gentle flexure and shoulder.The neck is cylindrical and occupies onethird of the total length.The aperture is fringed with a denticulated collarette which may flare slightly.The vesicle surface is covered with simple and branched spines which are aligned parallel to the longitudinal axis.The space between spines in a row is 4 to 8 μm, and their dimensions vary between 6 to 25 μm in length and 0.6 to 6 μm in width.No mucron or other basal features are present.
Description: The vesicle chamber is elongated ovoid, with a weak to inconspicuous basal margin and a convex base which, can be slightly concave in some instances.The flanks can be straight, but are mostly slightly convex.The flexure is weak to inconspicuous, and no shoulder is present.The neck is cylindrical and occupies one-third (30%) of the total length.The aperture is fringed with a simple and gently flaring collarette.The vesicle surface is covered with simple and bifurcated or multi-branching spines which are aligned parallel to the longitudinal axis.The space between spines in a row is c. 10 μm, and they vary between 2 to 35 μm in length and 1 to 6 μm in width.The base ornamentation appears to be equivalent to the rest of the surface.
Remarks: Ramochitina boliviensis Grahn, 2002 [7] differs from Ramochitina ramosi (Sommer & van Boekel, 1964) [74] by its longer, more sparsely distributed and variable spines, and in addition in R. ramosi the spines are always bifurcated.R. boliviensis may display different types of ornamentation in one specimen; the spines can be simple, bifurcated or branching several times near the base or at their tips.Additionally, on some occasions simple, stout, tapering spines can also occur over the vesicle surface.
The specimens recovered from the Tacobo borehole display the same vesicle shape, measurements and variability of spines described for the species.The badly preserved specimens were assigned to the species with some doubt.In these cases, the spines are completely broken,  but the sparse alignment of the ornamentation can be observed through the attachment scars, and the vesicle shape and measurements coincide with those for R boliviensis.
Ramochitina Derivation of the name: Following the original nomen nudum etymology described by Pe ´rez-Leyto ´n [9 p.172], being named after the La Candelaria section (Department of Chuquisaca, Bolivia).Note that this was following a more detailed study of the material as analyzed by Paris et al. [76].
Diagnosis: Ramochitina species with a subcylindrical to ovoid vesicle chamber with aligned spines which may be simple, lambda or multirooted.
Description: The vesicle chamber is subcylindrical to ovoid, with a broadly rounded basal margin and a convex to flat base.The flanks are slightly convex to straight, with a gentle flexure and very weak to inconspicuous shoulder.The short neck is cylindrical with a non-flaring collarette and occupies one-third (30%) of the total length.The ornamentation which covers the surface consists of densely aligned spines spaced between 2 and 5 μm.The spines may be simple, lambda or multirooted and they measure between 2 to 20 μm long and 0.5 to 4 μm in width (Fig 7F ').The base bears similar but smaller ornamentation compared with the rest of the vesicle wall.
Remarks: Pe ´rez-Leyto ´n [9 p. 296] described as diagnostic features "Ramochitina with cylindrical-ovoid chamber, short neck, flat bottom, ornamented by simple, thin, very densely arranged spines".However, in the detailed description, they mentioned that the base could be either flat or slightly convex due to flattening.~1/3 to 1/4 of the total of the vesicle).The slender outline and ornamentation are the main diagnostic features of this species.The variety of the spines is not mentioned as the main characteristic, but they are detailed in the description.In the material recovered herein, the variety of the spines seems to be an important characteristic and is thus considered a diagnostic feature for the taxon as formally described herein.
Ramochitina callawayensis (Urban & Kline, 1970) [77], resembles Ramochitina candelariaensis sp.nov., but they differ in the latter having a larger overall size and smaller neck (in R. candelariaensis the neck occupies 1/3-1/4 of the total length, meanwhile in R. callawayensis the neck occupies 2/5 of the total length).The size of the spines is similar; however, the ornamentation is much denser in R. candelariaensis sp.nov.Ramochitina implicationis (Urban, 1972) [59] differs from our species in having a cylindro-spheroid chamber shape, smaller overall size and shorter spines, which are usually bifurcated.Ramochitina jutaiense Grahn et al., 2003 [78], has an ovoid elongated body and the ornamentation consists only of multirooted and simple spines, meanwhile, R. candelariaensis bears more diverse ornamentation.
Description: The vesicle chamber is ovoid, with a broadly rounded basal margin and a convex base.The flanks are straight with a weak convexity in some specimens.The flexure is inconspicuous, and the shoulder is absent.The short neck is cylindrical with a non-flaring collarette and occupies one-third (30%) of the total length.Densely aligned spines ornament the vesicle surface (space between spines c. 5 μm).The spines may be simple or multi-branching at the base and the measurements vary between 5 to 27 μm long and 0.5 to 3 μm in width.Basal features could not be discerned due to the inward flattening of the specimens.
Remarks: The specimens assigned herein to Ramochitina stiphrospinata Grahn & Melo, 2005 [39], have the same vesicle outline and ornamentation characteristic of the species.Grahn and Melo [39] mentioned that the specimens of R. stiphrospinata from the Parnaı ´ba Basin are smaller than those described by Lange [79] and Grahn et al. [64] in the Parana ´Basin -the material recovered herein, however, bears the same dimensions as those previously recorded for the Parana ´Basin by Grahn and Melo [39].Occurrence: Global distribution: Ramochitina stiphrospinata Grahn & Melo, 2005 [39] was recorded in Northern Gondwana from the early Givetian of Well A1-37 in Libya [61].Western Gondwana: from the early Givetian of the São Domingos Formation, Parana ´Basin, Brazil [64,79] and the early Givetian of the Pimenteira Formation, Parnaı ´ba Basin, Brazil [39]; from the Givetian of the Huamampampa Formation, Bolivia [7]; from the Eifelian to Givetian of the Los Monos and Iquiri formations, Northern Argentina and Bolivia [67].
Material: One well-preserved specimen was observed and measured with an SEM from sample 9163 (5035 m) (Table 23).
Diagnosis: Ramochitina species with a subcylindrical vesicle chamber and multirooted spines connected at the end forming irregular ridges parallel to the longitudinal axis.
Description: The vesicle chamber is sub-cylindrical, with a weakly rounded basal margin and a convex base.The flanks are slightly convex with a distinct flexure and a very weak shoulder.The neck is cylindrical and the aperture bears a simple non-flaring collarette.The ornamentation consists of birooted, multirooted and coalescent spines aligned with the axis of symmetry of the vesicle (Fig 8L ').The spines are between 7 to 18 μm long, 1 to 3 μm in width and they reach 12 μm in examples that are coalescent.The ornamentation on the base could not be discerned, as a consequence of inward flattening.
Remarks: Ramochitina cf.durandii n. n. bears resemblance to R. durandii n. n.Pe ´rez-Leyto ´n, 2007 [9] in terms of vesicle size, but they differ in the vesicle shape since the latter is elongated-conical and our specimen has a sub-cylindrical chamber.In the original description, Pe ´rez-Leyto ´n [9] mentioned two types of ornamentation; one over the vesicle surface, and the other one over the basal margin.Although the spines on the vesicle wall are very similar to the multirooted spines connected at the end, which form irregular ridges parallel to the longitudinal axis, described for R. durandii n. n. by Pe ´rez-Leyto ´n [9], the basal margin ornamentation was not observed in the Tacobo specimen.These coalescent and low-density rows of spines have not been found in other species of the genus, but due to the similarity with one of the ornamentations described for Ramochitina durandii n. n. it is considered here to be a species with affinity to the aforementioned taxon.Since only one specimen was recovered herein, open nomenclature is preferred for the Tacobo material.
Occurrence: Global distribution: Ramochitina durandii n. n.Pe ´rez-Leyto ´n, 2007 [9] is restricted to Western Gondwana.It was described from the Los Monos Formation in the Middle Givetian of south Bolivia.
Diagnosis: Ramochitina species with an ovoid vesicle chamber and small interconnected spines aligned in rows.Description: The vesicle chamber is ovoid, with a broadly rounded basal margin and a convex base.The flanks are convex with a marked flexure and weak shoulder.The neck is cylindrical with a non-flaring denticulate collarette, and occupies one-half (50%) of the total length.The ornamentation consists of densely aligned small spines covering the vesicle surface (space between spines c. 4 μm).The spines may be simple or multirooted and the measurements vary between 1 to 8 μm long and 0.6 to 2 μm in width.Basal features could not be discerned due to the inward flattening of the specimens.
Remarks: Open nomenclature is retained for the specimens herein, due to the paucity of material, and as there are no Ramochitina species recorded previously bearing these small interconnected spines aligned in rows.If more examples are found through future studies, it could potentially be erected as a new species.
Occurrence: Ramochitina sp. is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Description: The vesicle chamber is cylindro-conical, with a well-rounded basal margin and a flat to slightly concave base.The flanks are mostly straight, though they may display a weak concavity or convexity.The neck is cylindrical with a non-flaring collarette and occupies one-third (35%) of the total length.The vesicle surface is generally smooth, but some tubercules (less than 2 μm in height) may be present sparsely.Above the basal margin, a constriction is present, which can be more evident in some specimens due to differences in flattening.The base bears a crown of simple and short processes (up to 35 μm long).Similar smaller spines are present around the neck in some specimens.

Remarks:
The specimens recovered herein all display the characteristic vesicle shape and the constriction above the basal edge.The basal processes display some degree of damage but when they are complete, they are consistent with those of A. biconstricta.In the incomplete specimens with basal processes broken or badly preserved, the distinctive vesicle chamber outline and the characteristic constriction near the base allow for tentative assignation to this species.
There are many discussions about this species.It was originally assigned to the genus Conochitina, and has been placed subsequently within Cladochitina, Spinachitina and finally to Ancyrochitina.Lange [80] described the species as Conochitina biconstricta whose main distinctive characteristic is a constriction on the vesicle chamber above the base; however, he mentioned that some specimens may not display this feature due to excessive flattening.Lange [79] later created the genus Cladochitina in 1967 and renamed the species in question as Cladochitina biconstricta.The species Ancyrochitina parisi erected by Volkheimer et al. [81] could be referring to those specimens without the constriction over the base mentioned by Lange.For some authors, A. parisi would be a junior synonym for A. biconstricta.Grahn in Grahn et al. [64,82] divided Ancyrochitina biconstricta into three groups: Ancyrochitina sp.A, Ancyrochitina parisi, and Spinachitina biconstricta, nonetheless in a later revision from 2011 [83] he considered Conochitina biconstricta Lange, 1949 [80] and Cladochitina biconstricta Lange, 1967 [79] as synonyms of Ancyrochitina biconstricta.Note that the genus Cladochitina Lange, 1967 [79] was considered a junior synonym of Spinachitina Schallreuter, 1963 [84] by Paris et al. [23 p. 596], and is thus not used in current chitinozoan nomenclature.
In the Tacobo borehole, we distinguish herein specimens with and without the constriction above the base and we consider them to be two different species.Every specimen assigned to Ancyrochitina biconstricta herein displays this characteristic feature.
Description: The vesicle chamber is conical, with a broadly rounded basal margin and a flat to slightly convex base.The flanks are generally straight but may display a weak concavity.The flexure is inconspicuous and the shoulder is absent.The neck is cylindrical and occupies one-third (35%) of the total length.The aperture is fringed by a collarette which may flare gently and the vesicle surface is smooth.The basal margin bears a crown of 6 to 8 stout simple processes of conical shape, extending up to 50 μm long, and 16 μm in width at the base.Similar spines can be distinguished around the neck in some specimens.
Remarks: Although some material is damaged, the specimens recovered herein possess the typical well-developed short neck and simple short processes (that widen at the base and are sharply curved at their tips) that are characteristic of this species.The Tacobo material herein resembles closely specimens described from the Cedar Valley Formation (USA) by Collinson & Scott [57], and from Brazil by Grahn and Melo [39].
Urban [59] suggested that their Ancyrochitina megastyla and Earlachitina (now Ancyrochitina) latipes (Collinson & Scott, 1958) [57], were variations of Ancyrochitina cornigera.As our material herein is comparable to the type material of A. cornigera Collinson & Scott, 1958 [57], we hold to this assignation.The first appearance of this species is used to define the base of the A. cornigera Interval Range Biozone of the middle Givetian [87].
Ancyrochitina flexuosa Almeida-Burjack, 1996 [86] .1996Ancyrochitina flexuosa; Almeida-Burjack, p. 64, pl.Material: Nine well-preserved specimens, and two assigned with uncertainty, were observed and measured from samples 9162 (4975 m) and 9164 (5065 m) (Table 27).Description: The vesicle chamber is conical to subconical, with a sharply rounded basal margin and a flat to slightly convex base.The flanks are straight to slightly convex with a gentle flexure and inconspicuous to absent shoulder.The neck is cylindrical and occupies two-fifths (40%) of the total length.The aperture is surrounded by a weakly flaring simple collarette and the vesicle surface is smooth.The basal margin displays 8 to 10 complexly branching (up to 5 th order) processes which are bent towards the aperture.The processes are between 20 and 60 μm long and up to 10 μm in width.The neck bears similar ornamentation, although the appendages are bent towards the base.
Remarks: Ancyrochitina flexuosa Almeida-Burjack, 1996 [86] is distinguished from other Ancyrochitina species by its ramified processes occurring in a crown around the base.The processes are bent towards the aperture and constitute the main branch from which secondary ramifications divide subsequently up to the fifth branching order.Similar spines surround the neck, but they are bent towards the base.The specimens recovered herein bear the distinctive processes and are of the same size and shape as A. flexuosa Almeida-Burjack, 1996 [86].
This species needs further revision considering that other species have been created with the same characteristic vesicle shape and processes as A. flexuosa, but with different dimensions.Almeida-Burjack [86] originally described and measured 29 specimens from the Parana Basin with a total length between 136.5 and 163 μm.Grahn [7] created the species Ancyrochitina postdesmea, from south-central Bolivia and he mentions that the main difference with A. flexuosa was the larger size, as the total length of the specimens measured therein was between 180 and 219 μm.However, Grahn and Melo [39] found smaller specimens of A. postdesmea in the Parnaı ´ba Basin (125-140 μm long).As a consequence of this, Grahn [83] recognized that the two species were the same and he considered A. postdesmea to be a junior synonym of A. flexuosa.Another author, Pe ´rez-Leyto ´n [9] identified a new species, Ancyrochitina yeserae (nomen nudum) from the early Givetian in southern Bolivia.The main characteristic was the small vesicle size (125 to 142 μm) and the strong multi-furcated appendages at the base and around the neck.He remarked that the main difference between A. yeserae and A. postdesmea Grahn, 2002 [7] was the overall length.We consider herein that A. postdesmea and A. flexuosa are synonymous (with the latter taking precedence), and A. yeserae to be a junior synonym of this taxon (see synonymy list), as the size range fits within that described by Grahn [7] and Grahn and Melo [39] (i.e.136.5 to 219 μm).
Occurrence: Global distribution: Ancyrochitina flexuosa Almeida-Burjack, 1996 [86] was recorded in Euramerica from Canada in the early Givetian of the Hamilton Formation, southwestern Ontario [88].Western Gondwana: from the Givetian of the Ponta Grossa Formation, and early Givetian of the São Domingos Formation, Parana ´Basin, Brazil [64,86], and from Bolivia in the early to middle Givetian of the Huamampampa and Los Monos formations [7], the latest Eifelian-earliest Givetian of the Los Monos Formation [8], and the lower to middle Givetian of the Tatarenda X27 and Camiri 201 boreholes [9].
Description: The vesicle chamber is subconical, with a broadly rounded basal margin and a flat base.The flanks are straight to gently convex, with a weakly developed flexure and inconspicuous to absent shoulder.The neck is cylindrical and occupies one-third (35%) of the total length.The aperture possesses a slightly flaring collarette that can be softly denticulated in some specimens.The vesicle surface is smooth.The basal margin bears a crown of 6 complexly multi-branching (up to 5 th order) processes which can reach 60 μm long and up to 8 μm in diameter.The neck holds smaller, similar ornamentation, but as a consequence of bad preservation, they are mostly fractured.
Remarks: The Tacobo specimens are more slender than Ancyrochitina flexuosa Almeida-Burjack, 1996 [86] and with spines not clearly developed.Although the material herein is scarce and highly damaged, the basal processes bear similar complex branching to A. flexuosa.The main ramification subdivides subsequently into a higher order of branching until at least the third order.However, the poor preservation of the material does not allow for the distinction of any other characteristic features, and so open nomenclature is retained for the specimens herein.
Description: The vesicle chamber is conical, with a sharply rounded basal margin and a slightly concave base.The flanks are straight with a gentle flexure.The neck is cylindrical with a non-flaring collarette and occupies one-half (50%) of the total length.The neck surface is entirely covered with simple and bifurcated spines which can reach up to 15 μm long; meanwhile, the rest of the vesicle surface is smooth.The basal margin displays 6 processes bifurcated at their tips, which measure between 15 and 40 μm long, and up to 7 μm in width near the base.Remarks: Although our material is scarce and not perfectly preserved, the main characteristic features of Ancyrochitina morzadeci Paris, 1981 [27] can be distinguished.The specimens herein have the same vesicle shape and slender outline, and the diagnostic narrow long neck with simple spines all over the surface.The basal processes are mostly fractured, although at least one of them is complete and displays the branching nature originally described by Paris [27].
Description: The vesicle chamber is conical, with a sharply rounded basal margin and a flat to convex base.The flanks are straight to convex with a gentle flexure and inconspicuous to absent shoulder.The neck occupies two-fifths (40%) of the total length and is cylindrical with a slightly flared collarette.The vesicle surface is entirely covered with simple long spines up to 20 μm long.The basal margin bears a crown of 10 to 12 multi-branching processes that can reach 50 μm long and 10 μm in diameter.
Remarks: Grahn et al. [64] reported specimens of Ancyrochitina aff. A. morzadeci with more convex flanks and smaller neck than Ancyrochitina morzadeci Paris, 1981 [27], and Grahn [83] mentioned also that those specimens could possibly be a different species but that the degree of damage and paucity of material did not allow for further clarification.
Our material strongly resembles those specimens assigned to Ancyrochitina aff. A. morzadeci by Grahn et al. [64].The main difference besides the shape of the vesicle chamber is the presence of the spines all over the neck and below the flexure.In A. morzadeci, the spines only are present on the surface of the neck and the rest of the vesicle chamber is smooth, only bearing the characteristic basal process over the basal margin.Another difference mentioned by Grahn [83] is the stratigraphical range since Ancyrochitina aff. A. morzadeci Grahn et al., 2002 [64] was recorded from the late Eifelian-middle Givetian, while Ancyrochitina morzadeci was reported from the late Emsian by Paris [27].This statement would not be accurate for our material, however, since we can distinguish both species in our samples.Nonetheless, we agree with Grahn [83] that the specimens assigned to Ancyrochitina aff.morzadeci are likely a different species, but due to the scarce material and the degree of damage, open nomenclature is preferred for this species.Occurrence: Ancyrochitina aff.morzadeci Paris, 1981 [27] is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Ancyrochitina Material: Ten well-preserved specimens, and seventy-three displaying varying levels of damage to the processes, were observed and measured from samples 9162 (4975 m) and 9163 (5035 m) (Table 31).
Description: The vesicle chamber is conical to sub-ovoid, with a broadly rounded basal margin and a generally flat to slightly convex base.The flanks are straight, but they may display a weak convexity or concavity.The flexure is weakly developed and the shoulder is inconspicuous or absent.The neck is cylindrical and occupies two-fifths (40%) of the total length.The aperture is enclosed by a thin-walled slightly flaring collarette that in some specimens can be denticulated.The vesicle surface is smooth and the basal margin bears a crown of 6 to 8 simple processes (up to 80 μm long and 15 μm in diameter).The neck has similar ornamentation with spines that can reach 15 μm long.
Remarks: Ancyrochitina parisi Volkheimer et al., 1986 [81], is very similar to Ancyrochitina biconstricta Almeida-Burjack, 1996 [86], the main difference being the absence of a marked constriction above the base (see discussion in A. flexuosa).The original description of this species mentions that constriction can be present in some of the specimens, but this feature seems to be a consequence of the difference in flattening and not an inherent characteristic of the species.A. parisi also differs from A. biconstricta in the nature of the basal processes which in the former can not only be simple but also bifurcated and usually more robust.Although our material is flattened and mostly damaged, diagnostic features are always noticeable in all of the specimens, in particular the very distinctive vesicle outline.
Ancyrochitina parisi Volkheimer et al., 1986 [81], was originally described from the uppermost Lower Devonian in the Puesto El Tigre Formation.The age was estimated because the strata, which contained this species, were lying immediately above those with Ramochitina magnifica and below those with Sphaerochitina pilosa (= Fungochitina pilosa).These authors proposed a local biozonation with Ancyrochitina parisi as an index species from the uppermost Lower Devonian.Grahn et al. [64,82], Grahn [7,10], and Noetinger and di Pasquo [66,94], recorded this species from the late Emsian, possibly including the earliest Eifelian, from Brazil,  32).
Description: The vesicle chamber is subcylindrical to conical, with a well-rounded to sharply rounded basal margin and a slightly convex base.The flanks are straight to gently convex with a flexure and shoulder that are inconspicuous.The neck occupies one-third (30%) of the total length and is cylindrical with a thin-walled slightly denticulated collarette around the aperture.The vesicle surface is generally smooth nonetheless; the body or the neck may bear some small randomly distributed tubercules (less than 2 μm in height).A crown of 6 to 8 simple long processes (mostly broken) can be recognized over the basal margin with measurements between 30 to 80 μm long and a diameter between 6 and 13 μm near the base.Broken processes and scars of similar nature are recognized around the neck in all specimens.
Remarks: The Tacobo borehole specimens assigned to Ancyrochitina cf.langei are mostly poorly preserved.Basal processes can be discernible, but most of them are fractured or  [74].The basal processes, when they are complete, display similar measurements and shape to those described for A. langei, and as such the main differentiation is the slightly bigger vesicle size of A. cf.langei herein.Previous records of A. langei from Bolivia [7] display strong similarities to our A. cf.langei material, even having the same degree of damage since broken processes are very common.Nonetheless, an open nomenclature is preferred for the Tacobo material.
Diagnosis: Ancyrochitina species with an ovoid vesicle chamber and very long processes which bifurcate delicately at their tips.
Description: The vesicle chamber is ovoid, with a broadly rounded basal margin and a flat to slightly convex base.The flanks are convex with a marked flexure and a distinct shoulder.The neck is cylindrical, but the aperture is fractured and no other features could be discerned.The vesicle surface is smooth and over the basal margin, a crown of 6 long processes can be distinguished, which are up to 115 μm in length and delicately bifurcated at their tips (Fig 8O').
Remarks: The specimen recovered herein is broken over the aperture, but the vesicle chamber features can be distinguished clearly.The ovoid vesicle shape is uncommon in the genus Ancyrochitina, but the presence of processes on the basal margin suggests this generic assignment.The long, thin, distally-bifurcated processes are not seen in any other Ancyrochitina species.Taxa belonging to the genus Plectochitina bear similar types of appendices, but they also should display a spongy texture that is not apparent in our specimen.Based only upon on a single broken specimen bearing these characteristics, it is retained in open nomenclature.
Occurrence: Ancyrochitina sp.A Is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Ancyrochitina sp.indet.Material: Seventy-six badly preserved specimens were observed from samples 9158 (4725 m), 9161 (4870 m) and 9162 (4975 m).Description: All the material bears broken remnants or scars where the processes were attached.The vesicles are generally broken and without a distinguishing outline that would allow for assignation to species level.The vesicle surface of all specimens is smooth without any ornamentation.No other features could be discerned.
Remarks: The specimens can be assigned with confidence to Ancyrochitina, based upon the presence of basal processes or attachment scars on the basal margin.All specimens bear a smooth vesicle surface and no diagnostic vesicle shape.Due to a lack of any further diagnostic features other than the basal processes, identification to species level could not be attempted.
Material: One flattened and broken specimen was observed and measured from sample 9163 (5035 m) (Table 34).
Diagnosis: Ancyrochitininae with a subcylindrical vesicle chamber with anastomosed basal process with a membrane in between.
Description: The vesicle chamber is subcylindrical, with a sharp basal margin and concave base.The flanks are straight and the flexure and shoulders are absent.The length of the neck cannot be discerned due to the broken aperture and the vesicle surface is smooth.The basal margin displays a crown of 6 coalescent processes with a thin membrane between them (Fig 8P ').
Remarks: Only one broken specimen could be assigned with uncertainty to the genus Clathrochitina based upon the nature of the basal processes.This specimen bears a thin membrane between the processes which is not present in Clathrochitina.Another difference are the processes that should be anastomosed, as on the specimen herein they are only coalescent near the basal margin.These two unusual features make assignation to Clathrochitina questionable.However, this is the only genus to date which is described with complex processes, and thus the only one we reasonably make an assignation to.If further similar specimens are found, it may be possible that a new genus is erected to contain specimens with this unusual combination of features.
Occurrence: Clathrochitina?sp. is recorded herein from the early Givetian in the TCB X-1001-Tacobo borehole.
Genus  Diagnosis: Alpenachitina species with a conical vesicle chamber, with multi-branching processes aligned in two rows and the neck covered with simple spines.
Description: The vesicle chamber is conical, with a sharply rounded basal margin and a flat to slightly convex base.The flanks are straight with a gentle convexity and the flexure and shoulder are weakly developed to inconspicuous.The neck occupies two-fifths (40%) of the total length and is cylindrical with a non-flaring and slightly denticulated collarette.The vesicle surface is smooth.The ornamentation consists of 2 rows of processes, one over the basal margin and the other one below the shoulder.The processes are long (up to 50 μm) and mostly multi-branching at their tips, while some of them may bifurcate near the base.The neck is entirely covered with simple and branching spines.
Remarks: The specimens herein bear the main characteristic of the genus Alpenachitina, in that they possess processes and spines arranged in three well-differentiated rows.However, the nature of these processes and the vesicle shape is different to any other Alpenachitina species.
A. matogrossensis and A. petroviensis have elongated and tubular processes with different branching at their tips and coalescent processes, and A. eisenacki bears stout multi-branching spines.The processes upon our specimens differ in that all of them are either simple or show less complex multi-branching at their tips.Due to the paucity of specimens they are retained in open nomenclature.
Occurrence: Alpenachitina sp. is recorded herein from the early and middle Givetian in the TCB X-1001-Tacobo borehole.

Definition of biozones
Chitinozoan biozonations are usually proposed over well studied sections as interval zones.When the material studied comes from outcrops and drilling cores, the base of the biozones are stablished with the first occurrence (FAD) of and index species until the FAD of another index species [4,87].This is because the occurrence of the specimens are certain to be precise and not to be material coming from other stratigraphical levels.When we work with cutting material the occurrence of the specimens may be from the bearing stratigraphical level or may be from some younger beds due to the caving process from the drilling.In that case, for biostratigraphical studies the last appearance (LAD) of an index species are usually used instead of the FAD.However, if we only use the LAD, the difficulty relies on the comparison and correlation with the currently existing biozones.Therefore, we decided to treat our material with the same methodology that is used for other biozonations, knowing that the limits proposed for our biozones may be not precise and may need further adjustment in the future (Fig 9).

The Ramochitina candelariaensis-stiphrospinata
Local Assemblage Biozone.The Ramochitina candelariaensis Local Assemblage Biozone was originally described by Pe ´rez-Leyto ´n [9] as an almost monospecific biozone together with Ancyrochitina sp.aff. A. biconstricta.In the Tacobo borehole this biozone can be recognised in the lowermost samples (9116-9166) and is characterized by the FAD of Ramochitina candelariaensis sp.nov.and Ramochitina stiphrospinata until the FAD of Ancyrochtina flexuosa.
Age: late Eifelian-early Givetian.In the Tacobo borehole this biozone is recognised in the uppermost sample (9151) and is characterised by the presence of Fungochitina pilosa and the LAD of Ramochitina boliviensis.Species such as Angochitina galarzae and Ramochitina autasmirimense can also be recognised in this biozone.

Age and correlation (Fig 10)
The Los Monos Formation is considered to be a diachronous unit as a consequence of the physiography of the basin [2].This is inferred by the discrepancy in age proposed by different authors.In South Central Bolivia an early to middle Givetian age for the lower part of the Los Monos Formation extending to the early Frasnian at the top was proposed by Grahn [7].In the Central South Subandean of Bolivia, an early to late Eifelian age was suggested by Troth et al. [20].In the Sub-Andean of Bolivia and Argentina, the age inferred was late Eifelian-early Frasnian [67] and later restricted to late Eifelian-early middle Givetian [19].For the TCB X- 1001-Tacobo borehole, Garcı ´a Muro et al. [3] proposed an age of Eifelian?-EarlyGivetian to the Middle Givetian for the Los Monos Formation based on miospores and organic-walled phytoplankton, coinciding with the age proposed herein.Although, it does not provide a tighter constraint for the dating of this unit.Coincidently with the study therein, the stratigraphical distribution of the chitinozoans was analysed based on the last appearance data (LAD) from the bottom to the top of the borehole since the palynological assemblage comes from cutting samples.
The stratigraphical range of the Sphaerochitina ricardi and Ancyrochitina morzadeci coincides with the early Givetian age of the bearing samples (9166 to 9161).The A. morzadeci early Givetian records are from Western Gondwana [7,39,64], though herein is the first record of S. ricardi for Western Gondwana.Even though Ramochitina durandii is middle Givetian in age [9], it is assigned with some reservation to the early Givetian in the Tacobo borehole.Angochitina galarzae has a wide stratigraphical range from the Givetian to the early Frasnian [30,66,67] which coincides with the early and middle Givetian assigned to the TCB X-1001-Tacobo borehole samples (9164 to 9158).
Ancyrochitina parisi is restricted to Western Gondwana and was originally described in the uppermost Lower Devonian of northern Argentina [81].Its stratigraphical range is currently restricted to the late Emsian-earliest Eifelian [7,66,82,94] and the first occurrence of this species defines the beginning of the Ancyrochitina parisi Interval Range Zone for the late Emsian in the Western Gondwanan chitinozoan biozonation [10].Considering Troth et al.'s [20] record of C. varispinosa as A. parisi (see systematic discussion of the species), the last occurrence of this taxon would be in the early Givetian.
The species mentioned above are not useful to further constrain the age of the samples from the Tacobo borehole.Nevertheless, if taxa such as Ramochitina candelariaensis and Ramochitina stiphrospinata in the lowermost part of the borehole (9116-9166) and specimens of Ancyrochitina flexuosa in sample 9164 were not caved, this part could be assigned to the candelariaensis-stiphrospinata Biozone.This biozone correlates partially with the monospecific candelariaensis Bolivian local biozone proposed by Pe ´rez-Leyto ´n [9].The absence of Ramochitina candelariaensis in sample 9166 would suggest a possible postdesmea-yeserae Biozone age for this sample.However, the Tacobo candelariaensis-stiphrospinata Biozone is characterised by the presence of both species until the FAD of Ancyrochitina flexuosa (A.postdesmea-A.yeserae) which coincides with the Ramochitina stiphrospinata Total Range Zone for Western Gondwana [10].In this case, samples 9116-9166 could be assigned to the late Eifelian-early Givetian and would agree with the phytoplankton and miospore age proposed for this part of the borehole by Garcı ´a Muro et al. [3].
The presence of A. biconstricta, A. flexuosa and R. boliviensis would correspond to the early Givetian flexuosa local biozone which correlates partially with the postdesmea-yeserae Bolivian local biozonation [9]. A. postdesmea and A. yeserae are considered synonyms of A. flexuosa and this could explain why they are usually found associated.
The flexuosa biozone can be correlated with the Fungochitina microespinosa-Ancyrochitina taouratinensis from the Western Gondwana biozonation [10], however, it also could have some species in common with the R. stiphrospinata biozone according to Pe ´rez-Leyto ´n [9].
A. biconstricta was proposed as the index species of the middle Givetian A. biconstricta Local Assemblage Biozone of Bolivia [9].Nevertheless, it is mentioned that this biozone is difficult to correlate with others since the species need revision (see discussion for A. biconstricta).Therefore, this taxon should not be used as an index species, and consequently it is not used herein.
Fungochitina pilosa is a common Middle-Late Devonian species with worldwide distribution [57][58][59][60]62,[64][65][66].In Western Gondwana, this species has been used together with Ancyrochitina langei in the Parana ´Basin as the main species of a concurrent range zone for the late Givetian [64].In the Bolivian biozonation it is also considered the index species of the Fungochitina pilosa Local Assemblage Biozone for the middle to late Givetian [9].
In the Tacobo borehole the occurrence of F. pilosa and the LAD of Ramochitina boliviensis in sample 9160 would suggest a pilosa biozone and possible middle Givetian age for the uppermost 9158 sample.
Ancyrochitina cornigera is a well-known species with worldwide distribution used by Paris et al. [87] to define the base of the middle Givetian in a global biozonation.However, it has been recorded not only in the middle Givetian [57,59] but also from the early Givetian [60,88] 6.The current chitinozoan biozonation for Western Gondwana [10] should be revised, taking into account the Bolivian chitinozoan biozonation [9] and the new biozonation proposal based on the present study.

Fig 1 .
Fig 1. Location map of the Tacobo X1001 borehole.A: Location map of Bolivia in South America.B: Location map of the Tacobo borehole in Bolivia.C: Geological Province of the Subandino (SAP) of Bolivia.https://doi.org/10.1371/journal.pone.0297233.g001

Fig 2 .
Fig 2. Schematic stratigraphic column of the TCB X-1001-Tacobo borehole.Depths of the Los Monos Formation palynological samples are indicated by meters below ground surface (mbgs) and the number of the sample.https://doi.org/10.1371/journal.pone.0297233.g002

5.1. 2 The
Ancyrochitina flexuosa Local Assemblage Biozone.This biozone was originally described as the Ancyrochitina postdesmea-Ancyrochitina yeserae n. n.Local Assemblage Biozone [9].It is characterised by the simultaneous occurrence of Ancyrochitina postdesmea and Ancyrochitina yeserae n. n.Other taxa present in the lower part of this biozone are Ancyrochitina arirambaensis, Ancyrochitina biconstricta, Ramochitina boliviensis and Ramochitina devonica, and Ancyrochitina langei and Ramochitina autasmirimense for the upper part of this biozone.In the Tacobo borehole this biozone is erected as the Ancyrochitina flexuosa Local Assemblage Biozone, and can be recognised from sample 9164 until sample 9160.It is characterised by the FAD of Ancyrochtina flexuosa (synonymised with Ancyrochitina postdesmea and Ancyrochitina yeserae n. n.) until the LAD of Ramochitina boliviensis.Species such as Ancyrochitina biconstricta, Ancyrochitina cornigera and Lagenochitina tacobensis have also their FAD in this biozone.Age: early Givetian.5.1.3The Fungochitina pilosa Local Assemblage Biozone.Erected by Pe ´rez-Leyto ´n [9], this biozone for the middle-late Givetian is characterised by the presence of Fungochitina pilosa and a variety of Ancyrochitina species such as Ancyrochitina biconstricta, Ancyrochtina monosi n. n., Ancyrochitina postdesmea, Ancyrochtina yeserae n. n., and Ancyrochitina cf. A. taouratinensis.

Fig 10 .
Fig 10.Chitinozoan ocurrences in the TCB X-1001-Tacobo borehole.The black circles represent positively identified species and the blue circle open nomenclature species.The gray circles are species restricted to the TCB X-1001-Tacobo borehole.The first group of species have their first record in Western Gondwana.The second group are species recorded in all the palaeocontinents, and the last group represents species restricted to Western Gonwana.https://doi.org/10.1371/journal.pone.0297233.g010